The present invention relates to methods for isolating water soluble polysaccharides having molecular weights of at least 50 kDa. The disclosed process is entirely aqueous and results in efficient production of these polysaccharides.
Mixed-linkage (1xe2x86x923), (1xe2x86x924) beta-D-glucans, referred to herein as beta-glucan, are the predominant cell wall components of grain endosperm, particularly oats and barley, and are a well-established antihypercholesterolemic agent. In the case of beta-glucan found naturally in oats, this effect has been acknowledged by the U.S. Food and Drug Administration (FDA). Beta-glucan also has immunostimulatory properties when applied topically to the skin. The biochemical mechanisms by which beta-glucan exerts its therapeutic effects are largely unknown.
Cereal grain seeds generally contain a small amount of beta-glucan, with oats and barley being recognized as the richest sources of this material. The naked oat seed, known in the art as a xe2x80x9cgroatxe2x80x9d, typically contains from 2-4% by weight beta-glucan, depending upon oat variety and other factors such as growing conditions. Barley seeds may typically contain twice as much beta-glucan as groats. Beta-glucan is generally found in higher concentrations in the outermost layers of the seed (i.e., the xe2x80x9cbranxe2x80x9d). Thus, oat bran is defined as containing a minimum of 5.5% by weight beta-glucan, and typically contains up to 6% or 7% by weight beta-glucan.
In order to receive an efficacious amount of beta-glucan for reduction of low density lipoprotein (LDL) and total serum cholesterol, the FDA recommends total beta-glucan ingestion of at least 3 grams daily. However, it is difficult and inconvenient for the average individual to obtain this recommended amount because of the inherently low beta-glucan content in products such as oatmeal, oat bran muffins or cooked barley. For example, in the case of oatmeal, which is simply rolled whole oats, one would have to consume up to 150 grams (dry basis) each day, an amount which most individuals would find extremely burdensome. Accordingly, there is a compelling need to provide a more concentrated form of beta-glucan so that consumers can conveniently ingest therapeutic amounts of this material.
Previous processes for concentrating beta-glucan from cereals such as oats or barley have proven impractical for commercial manufacturing processes because of high cost and/or low yields. The prohibitive cost associated with processes disclosed in the literature is almost always a consequence of reliance upon a precipitation step in which beta-glucan is removed from aqueous solution by an organic solvent, especially alcohols such as ethanol or isopropanol, the use of which entails high in-process losses and difficult reclamation. This is evidenced in the marketplace wherein beta-glucan concentrates are only sold as cosmetic ingredients, with prices typically greater than $100 per pound. There is presently no concentrated form of beta-glucan priced so as to be affordable for use as a dietary supplement or food additive. The most concentrated form of commercially available beta-glucan for nutritional purposes contains just 15% beta-glucan (Nurture(copyright) 1500, supplied by Nurture, Inc., Missoula, Mont.).
U.S. Pat. No. 5,518,710 to Bhatty discloses alkaline extraction of barley and oat bran, addition of an amylolytic agent to degrade starches, followed by precipitation of beta-glucan with a polar alcohol. Beer et al. (Cereal Chemistry 73:58-62, 1996) disclose a process for isolating beta-glucan in which oat bran concentrates are extracted in aqueous solution at alkaline pH, followed by dialysis, ultrafiltration or alcoholic precipitation. The material resulting from these processes had a beta-glucan content of about 60-65%. Westerlund et al. (Carbohydrate Polymers, 20:115-123, 1993) disclose a procedure for isolation of beta-glucan involving lipid extraction, enzymatic removal of starch and protein, and subsequent ethanol precipitation. EP 0 377 530 A2 discloses a process for the preparation of a beta-glucan-enriched grain for use as a food or food additive, in which oats are slurried in cold water, followed by rapid homogenization and screening of the slurry. U.S. Pat. No. 5,013,561 to Goering et al. discloses a process for recovery of various products, including beta-glucan, from barley. In this process, barley is milled, mixed with water, heated and centrifuged to remove insoluble material. The supernatant is then heated, and centrifuged to remove insoluble material, and the supernatant is subjected to ultrafiltration to remove soluble sugars and to concentrate beta-glucan solids.
Other water soluble polysaccharides are used as food thickening agents, emulsifiers and/or stabilizers. These include, for example, guar gum, pectin, xanthan gum, locust bean gum, carrageenan, agar, furcellaran, acacia (gum arabic), ghatti gum, pentosan, arabinogalactan and the like. Pectin is also used as a veterinary antidiarrheal agent.
Xanthan gum is a water soluble polysaccharide gum having a molecular weight of greater than one million produced by the bacterium Xanthomonas campestris and is composed of D-glucosyl, D-mannosyl and D-glucosyluronic acid residues and differing proportions of O-acetyl and pyruvic acid acetal. Xanthan gum is used in foods, non-foods and cosmetics as a stabilizer and emulsifying agent.
Pectin is a polysaccharide found in cell walls of all plant tissues and occurs naturally as the partial methyl ester of xcex1-(1xe2x86x924) linked D-polygalacturonate sequences interrupted with (1xe2x86x922)-L-rhamnose residues. The typical average molecular weight for commercial pectins is on the order of 100 kDa. Pectin is used as a thickening agent in jellies and similar food products.
Locust bean gum has a molecular weight of about 310 kDa and is obtained from the ground kernel endosperm of tree pods of Ceratomia siliqua L. Leguminosae (St. John""s bread). Locust bean gum comprises proteins, carbohydrates such as reducing sugars, sucrose, dextrans, pentosans, ash, fat, crude fiber and moisture. The carbohydrate portion is water soluble. Locus bean gum is used as a stabilizer, thickener and binder in foods and cosmetics; as a sizing and finishing agent in textiles; for fiber bonding in paper manufacturing; and as a drilling mud additive.
Guar gum has a molecular weight of about 220 kDa and is obtained from ground endosperms of Cyamopsis tetragonolobus Taub. L. Leguminosae. The water-soluble fraction is called guaran which consists of linear chains of (1xe2x86x924)-xcex2-D-mannopyranosyl units with xcex1-D-galactopyranosyl units attached by (1xe2x86x926) linkages. Guar gum is used in paper sizing; as a protective colloid, stabilizer, thickening and film forming agent for various foods; as a binding and disintegrating agent in tablet formulations; in pharmaceutical jelly formulations; in suspensions, emulsions, lotions, creams, toothpastes; in the mining industry as a flocculant; as a filtering agent; and in water treatment as a coagulant aid.
Carrageenan is a structural polysaccharide of the red seaweed (Rhodophyceae). The structure consists of alternating copolymers of xcex2-(1xe2x86x923)-D-galactose and (1xe2x86x924)-3,6-anhydro-D-galactose. Carrageenan is used as a gelling, emulsifying, and stabilizing agent and viscosity builder in foods and non-foods.
Agar is a polysaccharide complex extracted from the agarocytes of algae of the Rhodophyceae. It can be separated into a neutral gelling fraction, agarose, and a sulfated non-gelling fraction, agaropectin. The structure is believed to be a complex range of polysaccharide chains having alternating xcex1-(1xe2x86x923) and xcex2-(1xe2x86x924) linkages. Agarose is used as a substitute for gelatin, in making emulsions including photographic emulsions, gels in cosmetics; as a thickening agent in foods; in meat canning; in production of medicinal encapsulations and ointments; as dental impression mold base; as corrosion inhibitor; in sizing for silks and paper; in the dyeing and printing of fabrics and textiles; in adhesives; in nutrient media for bacterial cultures; and as a veterinary laxative for dogs and cats.
Furcellaran is a gum obtained from a seaweed of the Rhodophyceae, the red alga Furcellaria fastigiata, fam. Furcellariaceae, order Gigartinales. The gum is the potassium salt of the sulfuric acid ester of a high molecular weight polysaccharide consisting mainly of D-galactose, 3,6-anhydro-D-galactose, and the half-ester sulfates of these sugars; one sulfate group occurs for each three or four monomeric units, which are arranged in an alternating sequence of (1xe2x86x923) and (1xe2x86x924)-linked units. Furcellaran is used as a natural colloid, gelling agent, and as a viscosity control agent used primarily in food products, but also in pharmaceuticals.
Acacia or gum arabic is a dried gummy exudation from the stems and branches of Acacia Senegal (L.) Wiild., Leguminosae, or other African species of acacia which comprises various polysaccharides. It is used as a mucilage, excipient for tablets, thickener and colloidal stabilizer.
Ghatti gum is the gummy exudate from stems of Anogeissus latifola Eall., Combretaceae, abundant in India and Ceylon. The structure is a complex water-soluble polysaccharide comprising L-arabinose, D-galactose, D-mannose, D-xylose, D-glucuronic acid and traces of 6-deoxyhexose. It is used as an emulsifying agent in pharmaceuticals, oils and waxes.
Pentosans and arabinogalactans are plant polysaccharides (Oats: Chemistry and Technology, Francis H. Webster, Ed., Chapter 4, pp. 79-91; TCI America, p. 113).
Curdlan is a water soluble polymer isolated from the bacterium Al. faecalis var. myxogenes 10C3 and is composed exclusively of (1xe2x86x923)-linked xcex2-D-glucopyranosyl units. Curdlan is used in foods as a water-holding and stabilizing agent, and is useful as a binder in feed and in water purification.
There is a need for an entirely aqueous process for concentrating beta-glucan and other water soluble polysaccharides which is efficient, economical and produces highly palatable concentrates for use in foods, cosmetic and pharmaceutical formulations. The present invention addresses this need.
One embodiment of the present invention is an isolated water-soluble polysaccharide-enriched film, wherein the polysaccharide has a molecular weight of at least about 50 kDa, formed by evaporating a soluble polysaccharide-containing solution until surface film formation occurs. Preferably the polysaccharide is xcex2-glucan, guar gum, pectin, xanthan gum, other hydrocolloid gums, arabinogalactans, pentosans, carrageenan, locust bean gum, agar, ghatti gum, curdlan or synthetic derivatives thereof. Preferably, the polysaccharide is a soluble dietary fiber. In one aspect of this preferred embodiment, the polysaccharide is obtainable from plants, animals, yeast or bacteria. The polysaccharide-enriched film may be provided in combination with a food product. Preferably, the film is milled, powdered, dissolved or otherwise dispersed prior to combination with the food product The polysaccharide-enriched film may also be provided as a pharmaceutical composition in combination with a pharmaceutically acceptable carrier. The polysaccharide-enriched film may also be provided as a cosmetic composition. In one aspect of this preferred embodiment, the film is milled, powdered, dissolved or otherwise dispersed prior to combination with the food product, pharmaceutically acceptable carrier or cosmetic composition.
The present invention also provides a method for preparing concentrated water-soluble polysaccharide by evaporating water from an aqueous polysaccharide-containing solution, and allowing a solid film of concentrated polysaccharide to form on the surface of the solution. Preferably, the polysaccharide is guar gum, carrageenan, agar, furcellaran, xanthan gum, other hydrocolloid gums, pectin, locust bean gum, acacia, ghatti gum, pentosan and arabinogalactan. In one aspect of this preferred embodiment, the polysacccharide is beta-glucan. Preferably, the evaporating is performed while heating the water soluble polysaccharide-containing solution. Advantageously, the polysaccharide is a soluble dietary fiber.
The present invention also provides isolated beta-glucan having a molecular weight greater than two million daltons and a purity of at least about 60%.